Herokuへデプロイした時に「Could not detect rake tasks」エラーが出る

#事象
Railsで作ったアプリをHerokuにデプロイしたら下記のエラーが、ターミナルのログに表示。

```terminal
remote:  !
remote:  !     Could not detect rake tasks
remote:  !     ensure you can run `$ bundle exec rake -P` against your app
remote:  !     and using the production group of your Gemfile.
remote:  !     Activating bundler (2.0.1) failed:
remote:  !     Could not find 'bundler' (2.0.1) required by your /tmp/build_242d76def6eeeeae57603bbca28c0d6b/Gemfile.lock.
remote:  !     To update to the latest version installed on your system, run `bundle update --bundler`.
remote:  !     To install the missing version, run `gem install bundler:2.0.1`
remote:  !     Checked in 'GEM_PATH=/tmp/build_242d76def6eeeeae57603bbca28c0d6b/vendor/bundle/ruby/2.6.0', execute `gem env` for more information
remote:  !     
remote:  !     To install the version of bundler this project requires, run `gem install bundler -v '2.0.1'`
remote:  !
```

```
 ! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'https://git.heroku.com/limitless-eyrie-60171.git'
```

#原因
bundlerのバージョンか、Gemfileっぽい

参考記事
https://qiita.com/satouwork0316/items/a738392bcab03bac657a

#対処方法

Gemfileの記述？
試しに、以前デプロイに成功したアプリの`Gemfile`をコピペして上書き。

```Gemfile
source 'https://rubygems.org'
git_source(:github) { |repo| "https://github.com/#{repo}.git" }

ruby '2.6.2'

# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
gem 'rails', '~> 5.2.3'
# 20190622 bcrypt追加※パスワードハッシュ化
gem 'bcrypt', '3.1.12'
# 20190617 bootstrap追加
gem 'bootstrap-sass', '3.3.7'
# Use Puma as the app server
gem 'puma', '~> 3.11'
# Use SCSS for stylesheets
gem 'sass-rails', '~> 5.0'
# Use Uglifier as compressor for JavaScript assets
gem 'uglifier', '>= 1.3.0'
# See https://github.com/rails/execjs#readme for more supported runtimes
# gem 'mini_racer', platforms: :ruby

# Use CoffeeScript for .coffee assets and views
gem 'coffee-rails', '~> 4.2'
# Turbolinks makes navigating your web application faster. Read more: https://github.com/turbolinks/turbolinks
gem 'turbolinks', '~> 5'
# Build JSON APIs with ease. Read more: https://github.com/rails/jbuilder
gem 'jbuilder', '~> 2.5'
# Use Redis adapter to run Action Cable in production
# gem 'redis', '~> 4.0'
# Use ActiveModel has_secure_password
# gem 'bcrypt', '~> 3.1.7'

# Use ActiveStorage variant
# gem 'mini_magick', '~> 4.8'

# Use Capistrano for deployment
# gem 'capistrano-rails', group: :development

# Reduces boot times through caching; required in config/boot.rb
gem 'bootsnap', '>= 1.1.0', require: false

# 20190626 Jquery gem追加
gem 'jquery-rails'
gem 'jquery-ui-rails'

group :development, :test do
	# Use sqlite3 as the database for Active Record
	gem 'sqlite3'
  # Call 'byebug' anywhere in the code to stop execution and get a debugger console
  gem 'byebug', platforms: [:mri, :mingw, :x64_mingw]
end

group :development do
  # Access an interactive console on exception pages or by calling 'console' anywhere in the code.
  gem 'web-console', '>= 3.3.0'
  gem 'listen', '>= 3.0.5', '< 3.2'
  # Spring speeds up development by keeping your application running in the background. Read more: https://github.com/rails/spring
  gem 'spring'
  gem 'spring-watcher-listen', '~> 2.0.0'
end

group :test do
  # Adds support for Capybara system testing and selenium driver
  gem 'capybara', '>= 2.15'
  gem 'selenium-webdriver'
  # Easy installation and use of chromedriver to run system tests with Chrome
  gem 'chromedriver-helper'
	# 20190615追記
	gem 'rails-controller-testing', '1.0.2'
  gem 'minitest',                 '5.11.3'
  gem 'minitest-reporters',       '1.1.14'
  gem 'guard',                    '2.13.0'
  gem 'guard-minitest',           '2.4.4'
end

# 20190615追記
group :production do
  gem 'pg', '0.20.0'
end

# Windows does not include zoneinfo files, so bundle the tzinfo-data gem
gem 'tzinfo-data', platforms: [:mingw, :mswin, :x64_mingw, :jruby]
```

下記のコマンドをターミナルで叩いて、再度デプロイ

```terminal
gem install bundler -v 2.0.2
bundle update --bundler
```

デプロイ出来た！！（解決）